Sean O'Brien is refusing to concede the ground war to David Pocock when Ireland and Australia collide at Eden Park on Saturday morning.

O'Brien has been selected out of position at openside for the pivotal Pool C showdown after David Wallace was ruled out of the World Cup with a knee injury. The 24-year-old's explosive ball-carrying last season saw him crowned European player of the year, but he is more comfortable in the number six jersey.

It is a big challenge to take on the brilliant David Pocock, viewed by All Black great Josh Kronfeld as a better seven than Richie McCaw, at the breakdown. But O'Brien insists he will go toe to toe with the Wallaby when possible.

"I wouldn't say my role is strictly as a seven. If I get the chance to carry, I'll carry," he said. "I'll just go out there and play the way I usually do and hopefully that'll be enough. I've seen plenty of Pocock and he's a world-class player. He's in form and is a big threat.

"If the chance arises and I think it's the right decision I'll go in to compete with him. We'll have to combat them getting that quick ball and preventing them from getting on the front foot. You want to test yourself against the best and the Wallaby back row is perceived to be the best at the moment."

O'Brien will be making his World Cup debut after missing last Sunday's opening 22-10 victory over the United States with a knee injury. The Leinster back-row has spent the last four weeks on the sidelines and insists he is eager to make his presence felt in New Zealand.

"It's good to be back involved. Training has been sharp and the sessions have been short because of the six-day turnaround between games," he said. "It's not damaging at all the fact several players are coming back from injury.

"If we weren't playing last week we were doing extra work off the pitch, so it's not as if we've been sitting on our arses. There isn't a feel of tension around the camp, it's more one of excitement. Australia feels like one of the biggest game of my career. It's my first World Cup game. I get a bit of nervous energy but that's a positive thing."